Valve's probably going to be updating their policy for trading, and it won't be for the better. They plan on introducing an escrow period when items will not be available to either user just because either one of them does not have Steam Guard Mobile Authenticator. If you don't have a smartphone or are over your data limits you probably won't be able to trade.

Here's what some inactive text in their javascript says:

'What is Escrow?', After a trade has been accepted by both parties, if either party's account has not been secured by the Steam Guard Mobile Authenticator, then to protect against unauthorized trades, the traded items will be placed in escrow. During the escrow period, the items will not be available to either user. This allows users who have not secured their accounts to cancel any unauthorized trades and recover their items. Canceling all pending and in-escrow trades will place a trading hold on your account for a few days to prevent any further unauthorized attempts to trade away items.

Here is the petition to stop this from being enacted. If you have multiple email addresses you can vote multiple times [Note you don't have to donate, that's just the confirmation page that your vote has been cast]

Right now it has over 11k signatures, but that's just a drop in the bucket compared to Valve's playerbase so we need as many as possible. Spread the word.

Not only if you "don't have a smartphone", it also sucks shit for Windows Phone users like myself. There is no official Steam app of any kind available for WinPhones, let alone a mobile account authenticator.

Without strife, your victory has no meaning. Without strife, you do not advance. Without strife, there is only stagnation